The long-term goals of our laboratory are: a) to understand the molecular mechanisms that regulate cardiac gap junctions and b) to apply that knowledge to the development of pharmacologic agents able to modify gap junction regulation and function. It is generally accepted that the carboxyl terminal is the major regulatory domain of Connexin43 (Cx43) channels. Our previous work has led to the hypothesis that regulation of Cx43 results from the binding of the carboxyl terminal (Cx43CT) domain -acting as a gating particle- to the cytoplasmic loop (Cx43CL) domain, acting as a receptor for the gating particle. Cx43CT dimerization and other intermolecular interactions involving the same domain may also participate in the process. As a key player in the regulation of gap junctions, the Cx43CT domain is a target for chemical manipulation intended to modify function. Recently, we reported the identification of a peptidic molecule, dubbed "RXPE," that bound Cx43CT with micromolar affinity and partly prevented octanol-induced and acidification-induced uncoupling in pairs of Cx43-expressing N2a cells. Preliminary data presented in this application further show that RXPE binds cardiac Cx43, facilitates the synchronization of electrical activity in monolayers of neonatal cardiac myocytes, and prevents heptanol-induced conduction block in the same preparation. It is the overall objective of this project to use RXP-E as a platform in the development of a gap junction- based pharmacophore of known cellular/molecular action, and to assess its possible efficacy in the maintenance of electrical synchrony in cardiac preparations. Specific aims are: 1: To characterize the effect of RXP-E on the molecular events mediating Cx43 regulation. 2: To analyze the actions of RXP-E on the chemical regulation of cardiac gap junctions, and on the electrophysiology of single cardiac myocytes, and 3: To assess the effect of RXP-E on cardiac action potential propagation, susceptibility to conduction block and arrhythmogenesis. Overall, these studies may lead to a better understanding of the role of gap junctions in the development of ischemia-induced arrhythmias and their possible intervention by chemical means.